Microwave proximity sensor

ABSTRACT

The invention relates to a type of proximity sensor. In one form, two sinusoidal signals travel along two transmission lines near an object. When the distance between one or both of the lines and the object changes, the speed of travel of one or both of the signals changes. There is a correlation between the speed change and the distance, thus allowing one to infer distance from speed change. One way to measure the speed change is to measure the phase relationship between the two signals.

The invention relates the measurement of distances, using microwaveradiation.

BACKGROUND OF THE INVENTION

There exist numerous techniques for noncontact measurement of smalldistances on the order of one inch to small fractions of an inch. Thesetechniques may use capacitive, magnetic, optical, or acoustic effects,with each technique having particular advantages in a given situation.The choice of technique frequently depends upon the required accuracy,the operating medium, and upon environmental constraints.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a new and improveddistance sensor.

SUMMARY OF THE INVENTION

In one form of the invention, two sinusoidal electromagnetic signals aretransmitted along separate transmission lines. When an object is nearone of the lines, the speed of travel of the signal on that line ischanged. The amount of change is a function (generally nonlinear) of thedistance between the object and the transmission line. Measurement ofthe change allows one to infer the distance. The change in speed oftravel is determined through a phase shift of the sinusoidalelectromagnetic signal. Further, phase changes may also be used to inferchanges in the object geometry or in the surrounding medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one form of the present invention.

FIG. 2 is an exploded view of the sensor 25 in FIG. 1.

FIG. 3 is a cross-sectional view of a form of transmission linegenerally called a stripline. (Two parallel striplines are shown.)

FIG. 4 is another form of the sensor shown in FIG. 2.

FIG. 5 illustrates the construction of a sensor in which the hybridcircuits 15 and 42 of FIG. 1 are integrated into the construction of thesensor of FIG. 2.

FIG. 6 is a schematic of the microwave circuitry of in FIG. 1 or FIG. 5.

FIG. 7 is a typical calibration curve.

FIG. 8 indicates the geometrical motions for calibration curves.

FIG. 9 is typical of a set of calibration curves when lateral motion isallowed.

FIG. 10 indicates the performance when a typical complex object ismeasured.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 6 will be considered together. A listing of typical relevantcomponents is given in Table 1, together with suppliers from which theycan be obtained. The electromagnetic source 3 is a Gunn diode oscillatoroperating at 10.5 gigahertz (GHz) which provides an input signal on line6. All connections in FIG. 1 are coaxial lines suitable for thefrequencies employed.

TABLE 1

3--ESC Model 206 dielectric resonator stabilized oscillator or M/A-COMGunn MA86651A

15 and 42--OmniSpectra 2032-6348-00 Hybrid Coupler

39--OmniSpectra 2054-60002-00 line stretcher

51--OmniSpectra 2085-6017-00 detector

62--Line Tool Co. Model I-1 Micropositioner

47 and 49--Midisco MDC 1075 precision terminations

Microwave connectors are OSM or SMA types.

The signal splinter 15 is a microwave hybrid circuit which splits theinput signal into two signals carried by lines 18 and 21, the twosignals being 90° out of phase and of approximately equal amplitudes inthis implementation. Termination 47 absorbs incidental unwantedreflected signal.

The two signals are fed to the sensor 25 which contains elements 27 and28 which will be called sensing lines. At the present time, it issufficient to state that when an object, such as a fan blade in FIG. 1,is near one of the sensing lines, 27 for example, the propagation of thesignal along that sensing line is changed. The object causes a change inthe dielectric medium surrounding the line 27, thus causing a change inthe speed of propagation of the signal along the line.

The two signals, after passing along the sensing lines, exit the sensor25 on lines 33 and 36 shown in both Figures. The signals are recombinedin signal splitter 42, which is identical to splitter 15. The phaseshifter 39 is adjusted so that the phasor addition of the two signalsresults in an addition at port G and cancellation, or a null, at port F.This compensates for unequal path lengths in 18, 21, 25, 33, 36, as wellas imperfections in the splitters and elsewhere. The unwanted power atport G is absorbed in termination 49 or used to monitor the operation ofthe source 3. The null signal at port F is the useful signal output. Inthis implementation, it is rectified by video detector 51, amplified byamplifier 57, and displayed by meter or oscilloscope 12. Distancemeasurement using port F will now be described.

In the absence of any object near the sensing lines, phase shifter 39 isfirst adjusted for a null, ideally zero, signal at port F. Then, anyobject approaching one of the sensing lines will then create a phasechange in the signal on that line such that the signal level at port Fwill change. This resulting signal can be calibrated in terms ofdistance for a given object.

The Inventor has performed an experiment using the apparatus shown inFIG. 1 and has obtained the data shown in FIG. 7 and FIG. 9. In theexperiment, a stationary object, which can be fan blade 30 when the fanis not spinning, was moved toward and away from sensor 25 by cross-slidetable 62. Distance 64 is represented by the horizontal axis in FIG. 7.The Inventor points out one significant feature of FIG. 7, which is thatthe vertical axis is on a logarithmic scale. For example, point 137represents a voltage which is a hundred times greater than that at point136. The apparatus shown in FIG. 1 has thus been found useful inmeasuring distances, such as distance 64.

A more detailed description of sensor 25 will now be given. FIG. 3illustrates in cross section a typical stripline, although other formsof transmission lines such as those generally known as microstrip orslotline, for example, should also be useful. Independent striplines 70are sandwiched between conducting ground planes 73 and 74 and separatedfrom the ground planes by a dielectric material. The dielectric groundplanes may be air or any suitable insulator known in the art. A typicalseparation of the ground planes may be 0.050 inch, and the width of thestripline 70 may be approximately one-tenth this spacing, although widerexcursions are sometimes used.

FIG. 4 is an exploded view of a stripline of the type in FIG. 3. Asection 80 of one of the ground planes 74 has been eliminated. Theexposed striplines 70A and 70B correspond to sensing lines 27 and 28shown in FIG. 6. Consequently, when an object 30 is near one of thestriplines, the speed of propagation of the signal is changed in thatline, as mentioned above.

FIG. 2 shows an alternate form of the invention shown in FIG. 4. In abroad sense, the stripline of FIG. 4 is folded along dashed lines 83which correspond to edges 83A in FIG. 2. Stripline regions 70A and 70Bcorrespond to the sensing lines 27 and 28 shown in FIG. 6. Folded groundplane 73A corresponds to the flat continuous ground plane 73 in FIG. 4,while the two ground planes 74A in FIG. 2 correspond to thediscontinuous ground plane 74 in FIG. 4. Again, when an object 30 isnear stripline section 70A, propagation in the stripline is altered. Theobject 30 is present in the air near the stripline section 70A. It is,of course, recognized that FIGS. 2 and 4 show only the conductiveelements of the sensor 25, with the dielectric not shown. In addition,minor changes in geometry known in the art affecting impedance in orderto compensate for the bending and ground plane changes are not shown.

A detailed diagram of another form of the sensor 25 is shown in FIG. 5.The sensor 25 in FIG. 5 is similar to that in FIG. 3, but an addedfeature is that the function of signal splitters 15 and 42 in FIGS. 1and 6 is assumed by regions 15A and 42A of striplines 70. Theconfiguration sketched in the Figure is generally referred to as abranched-line coupler. Dielectric regions 90, 92, and 110 are shown inthis exploded diagram. The length 103 in FIG. 5 is shown greatlyexaggerated. In practice, 70A is separated from 73C by a distancecomparable to that between 70 and 73B.

A significant advantage of the sensor shown in FIG. 5 is the integrationof the sensing lines and signal splitters 15 and 42 into one module. Incontrast, the splitters are external to the sensor 25 in FIG. 1. Thus,if the input signal is represented by arrow 115 in FIG. 5, the signalsreaching points B and C will be equal and 90 degrees out of phase. If nochange occurs in 70A, then 42A will recombine the signals such that theyare in phase and add at G, and a null signal will be seen at F. If thesensor is designed and manufactured to precise tolerances, the need forthe external phase shifter 39 in FIGS. 1 and 6 is eliminated, or itsfunction may be assumed by trimming (i.e., modifying the geometry)adjustments. Further, the natural symmetry of construction will avoidmany environmental effects on performance.

The Inventor points out that it is not generally the actual phasedifference which is measured or even sought. It is the difference in thevoltage at point F in the presence of an object, as compared with thevoltage at the same point in the absence of the object which isgenerally useful as an end result of the foregoing argument. As FIG. 7shows, this voltage is a function of the distance 64 in FIG. 1 betweenthe object 30 and sensor 25. Further, this voltage is approximately alogarithmic function of the distance for sensors and objects tested.

The term distance has been used loosely in the discussion above. FIG. 8illustrates a cross section of the sensor of FIG. 2, taken along lines8--8. If an object 30 is moved to successive positions indicated byphantom blocks 30A-C, a voltage-distance plot resembling that of FIG. 7will be obtained. However, if the object is moved along a differentpath, such as that shown by phantom objects 30D-F, a different plot suchas that of FIG. 9 may be obtained, for various distances of closestapproach. The sensor is calibrated as described above for a given objectshapes and known path.

The relative geometry is easily inferred for many measurements. Forexample, if a gas turbine engine blade 120 in FIG. 10 traverses theregion near the sensor 25 as shown by the arrow 125. A signal 127 on theoscilloscope 12 is obtained which resembles that shown. The oscilloscopesignal gives a signature of the geometry of the blade. For example,peaks 130 correspond to what are called in the art as squealer tips 133on the blade 120, and valley 136 would correspond to region 139 on theblade 120. Thus, in the gas turbine art, the invention can be used tomeasure the clearance between turbine blades and a turbine shroud, or toobtain the signature of the blade tip. In the latter case, a deviationin signature by another blade can indicate a deviation in bladegeometry, which is useful in testing. The deviation can indicate damagein the blade. In the former case, clearance (i.e., a distance similar todistance 64 in FIG. 1) measurement can be useful in the control ofturbine clearance in gas turbine engines. For example, in U.S. Pat. No.4,230,436, issued to Samuel H. Davison, on Oct. 28, 1980, and assignedto General Electric Company, which is hereby incorporated by reference,a system for controlling the turbine clearance is described. The presentinvention can provide real-time, immediate information as to actualturbine clearance, as an input to the control system.

An invention has been described wherein an object present near one oftwo transmission lines changes the speed of propagation in the line,thus altering the phase of the signals in the lines. Measurement of thephase change, as by phasor addition of the signals when the object isabsent, and again when the object is at known positions, and comparingthe two added signals allows one to establish a calibration curve as inFIG. 7. Later measurements of the signals in the presence of the objectat an unknown distance, such as that at point 185 in FIG. 7, indicatesthat the distance to the object is distance 189.

FIG. 2 and 4 show ground planes 73, 73A, 74 and 74A separated fromstriplines 70A and 70B. As shown, the ground planes are insulated fromthe strip lines. As the discussion above indicates, the dielectrics ofFIG. 5 are not shown in FIGS. 2 and 4. However, it is clear from theselatter Figures that object 30 is present in the air (a dielectric)surrounding the striplines. Further, as FIG. 5 indicates, when viewed inlight of either FIGS. 2 or 4, the dielectric separating the striplines70 from the ground planes 73 and 74 need not be the same as that betweenthe object 30 (in FIGS. 2 and 4) and the striplines.

As discussed in connection with FIGS. 7 and 8, it is recognized that useof FIG. 7 only applies to a situation similar to the one under whichFIG. 7 was generated. A collection of such points, however, as theobject moves in a known manner, may be used to infer additionalinformation.

Numerous substitutions and modifications can be undertaken withoutdeparting from the true spirit and scope of the present invention.

For example, the term splitter has been used to describe elements 15 and42. However, any microwave power divider, hybrid, or 3 dB directionalcoupler can be used to accomplish an equivalent result, namely,providing signals of identical frequency with known magnitude and phaseat points 18 and 21, as well as combining the signals on lines 33 and 36in a known manner in order to measure the phase shift occurring alongsensors 27 and 28.

Also, a diode 51 was described in FIG. 1. However, other forms ofmicrowave demodulators or receivers can be used to accomplish therectification. In particular, a microwave mixer and superheterodynereceiver with logarithmic amplifiers can be used to accomplish animproved result. The latter combination provides a linear output whichincreases the dynamic range of the device.

What is desired to be secured by Letters Patent of the United States isthe invention as defined in the following claims.

I claim:
 1. Apparatus for responding to a change in a dielectric medium,comprising:(a) means for propagating a signal along a conductor near thedielectric medium; and (b) means for measuring changes in the velocityof propagation of the signal along the conductor in response to changesin the dielectric medium.
 2. Apparatus for inferring distance based on achange in a signal velocity, comprising:(a) a signal source; (b) meansfor providing at least two electromagnetic signals having firstmagnitudes and a fixed phase relationship; (c) sensing means forreceiving two of the signals and for changing the velocity ofpropagation of one or more of the signals in response to a nearbyobject; and (d) measuring means for measuring the change in velocity. 3.A method of inferring distance based on a change in signal velocitycomprising the following steps:(a) propagating two signals along twotransmission lines; (b) changing the relative velocity of propagation ofthe signals; and (c) measuring the change of (b).
 4. Apparatus forresponding to an object located near a transmission line, comprising:(a)means for propagating a signal along the transmission line; and (b)means for measuring changes in the velocity of propagation of thesignal, caused by the object, along the transmission line.
 5. A methodaccording to claim 3 in which the transmission lines of paragraph (a)remain constant in length during changes in the distance to be measured.